SPSB1 inhibition induces cancer immune evasion by modulating the KLF6/PD-L1 axis in non-small cell lung cancer cells.

Non-small cell lung cancer (NSCLC) remains among the most lethal malignancies, with aggressive progression and limited survival. The binding of programmed cell death 1 ligand 1 (PD-L1) to its receptor PD-1, which is expressed on T cells, functions as a critical checkpoint that undermines anti-tumor immune defenses. Despite encouraging anti-tumor efficacy, PD-1 inhibition yields only modest objective response rates in clinical practice. This limitation stems, in part, from an incomplete elucidation of the molecular circuitry controlling PD-L1 expression within the tumor microenvironment. In this work, we delineated a previously unrecognized regulatory axis that modulates sensitivity to PD-1 blockade in NSCLC, centering on SplA/ryanodine receptor domain and SOCS box containing 1 (SPSB1), a substrate-recognition subunit of a Cullin-RING E3 ubiquitin ligase. SPSB1 is markedly upregulated in NSCLC, and its genetic depletion suppressed NSCLC cell proliferation In vitro. Paradoxically, this deletion triggered PD-L1 upregulation, resulting in functional exhaustion of cocultured T cells In vitro, attenuated anti-tumor immunity in vivo, and diminished therapeutic efficacy in immunocompetent murine models. Notably, combined PD-1 mAb and SPSB1 knockdown markedly attenuated tumor growth in immunocompetent mouse models. Mechanistically, SPSB1 ubiquitinated Krüppel-like factor 6 (KLF6), destabilizing the transcription factor, thereby relieving KLF6-mediated promotion of PD-L1 transcription. Upstream, polypyrimidine tract binding protein 3 (PTBP3) bound SPSB1 3'UTR, stabilizing its mRNA. Rescue experiments confirmed that the KLF6/PD-L1 axis is necessary to transmit the signal triggered by SPSB1 loss. In conclusion, this work establishes that SPSB1 suppression combined with PD-1 blockade may be a potential combinatorial strategy for NSCLC management.